1. Field of the Invention
The present invention relates to a driving force transmission mechanism and an image forming apparatus.
2. Description of the Related Art
Recently, along with the improvement of image quality and speed in an image forming apparatus, if any rotary member included in a photosensitive element, a developing unit, a transfer unit, or the like has rotational fluctuation, uneven density in an image tends to occur. Therefore, rotational accuracy of such a rotating member, in particular, a shaft of a developing roller, which has the greatest impact on image quality, is strictly required.
As conventional art, there has been proposed that an output shaft (a driving shaft) on a driving source side is coupled to an input shaft (a driven shaft, which is a rotational driving force receiving part) on a developing unit side by using a triball-type constant velocity joint (for example, Japanese Patent Application Laid-open No. 2009-24829, Japanese Patent Application Laid-open No. 2009-30646, Japanese Patent Application Laid-open No. 2009-30645, Japanese Patent Application Laid-open No. 2009-41640, and Japanese Patent Application Laid-open No. 2010-121740). By using the triball-type constant velocity joint, the driven shaft on the developing unit side (the rotational driving force receiving part) can be rotated at the same speed as that of the driving shaft on the driving source side even if a deflection angle is generated therebetween, thereby forming a high quality image without defects such as uneven density.
When the triball-type constant velocity joint is used as described above; however, the following problems occur as illustrated in FIGS. 11A and 11B. In an image forming apparatus in which a constant velocity joint 100 is used, a tubular portion 101a is provided so as to project from a bottom wall 10b of a joint outer ring 101, while a shaft end portion 102a of a driven shaft 102 fits into the tubular portion 101a as illustrated in FIG. 11A.
In order to integrally rotate the outer ring 101 of the constant velocity joint 100 and the driven shaft 102, a flat surface 103 is provided on an inner diameter surface of the tubular portion 101a, while a flat surface 104 corresponding to the flat surface 103 is provided on an outer diameter surface of the shaft end portion 102a. 
Furthermore, in order to fit the shaft end portion 102a into the tubular portion 101a, a micro gap A needs to be provided between the inner diameter surface of the tubular portion 101a and the outer diameter surface of the shaft end portion 102a. However, such a micro gap A generates a so-called fitting backlash, which causes an inclination (i.e., shaft misalignment) B between the driven shaft 102 and the outer ring 101 of the constant velocity joint 100 as illustrated in FIG. 11B. Such shaft misalignment in turn generates banding on an image due to pitch variation.
Therefore, there are needs for a driving force transmission mechanism that causes no shaft misalignment, whereby rotational driving force from the driving source is stably transmitted to the driven shaft, and an image forming apparatus that can form a high quality image using the driving force transmission mechanism.